I notice that too, maybe the reason why bitcoin price drop every January is that many people are following the previous graph which makes January as selling month. For if you compare every graph from different years there are some similarities.
Last January fees were only $0.10 and transactions fees were livable and we didn't have hub banker take over using the lightning network
and we were not in such a big bubble.
Tulip's had bad weeks to until one day it didn't so was that in the charts too.